The invention relates to a rod of hard metal or ceramic with at least one helical bore extending through it, to a method of manufacturing the rod, whereby a mass of plasticized hard metal or ceramic is forced through a die and simultaneously twisted into a helix, and to an extrusion die aligned with a mandrel for carrying out the method.
Rods of hard metal or ceramic with at least one helical, or twisted, bore extending through it are known, from EP 0 118 035 A1, German 3 600 681 A1, and U.S. Pat. No. 4,059,031 for example. Rods of this type are processed into drill bits, with the bores constituting channels to convey a rinse or coolant. The helical channels are created by appropriate spinners that twist the blank leaving the extruder at a rate calculated in relation to the mass's rate of flow to produce the desired geometry and pitch. This procedure demands, in addition to the extrusion die itself, spinners and appropriate controls. To eliminate these accessories, the aforesaid German 3 600 681 discloses twisting the mass into a helix during the actual extrusion. Extruding and twisting are accordingly carried out simultaneously in a single step. A die with at least one helical web extending in the direction of pressure around its inner surface imposes a radially inward torsion on the billet.
The mandrel in this known extruder terminates downstream of the inside of the die and has one or more elastic pins secured to it that extend into the die. The pins have the diameter of the desired channels. This known method of and device for extruding such rods, however, have several drawbacks. First, the torsion imposed on the billet by the helical webs on the inner surface of the die more or less decelerates radially inward from the billet's outer surface in accordance with the viscosity of the mass and its friction against the die. It is accordingly almost impossible to obtain inner bores with the desired geometry. Although a reproducible torsion is obtainable near the surface of the billet, in the vicinity of the webs, they do not extend very far in. The webs also make it impossible to obtain rods with a smooth surface, and the products exhibit definite undulations. Finally, the extruded ceramic or hard metal is highly abrasive. The webs wear out rapidly and the die must be changed frequently. Regrinding the inside of the die is expensive and increases the cost of the rods.
With the aforesaid state of the art as a point of departure, the object of the present invention is to provide a blank in the form of a rod of hard metal or ceramic with precisely twisted inner bores of a precise tolerance and constant pitch. The surface of the blank will also be smooth enough to eliminate the need for grinding often encountered with known rods to remove the typical undulations. The rods with their twisted inner bores will also be thin enough to reduce the waste of material when the blanks are processed into bits. The manufacturing process will be simpler, more reliable, and less expensive. Another object of the invention is a device for carrying out the method, specifically an extruder, that is simple and compact, that will last, and that will generate a precisely reproducible torsion. The spinners and associated controls employed for further processing into hard-metal or ceramic bits and the complicated dies with webs on their inner surface will no longer be needed.